1. Field of the Invention
The present invention relates to a method for preparing a fluorinated material usable as electrode active material, as well as the material obtained.
2. Technological Background
Lithium batteries using a lithium ion insertion compound as the basis for operation of the positive electrode are known.
Among the known insertion compounds, we may notably mention the lithium transition-metal oxides, for example LixCoO2, 0.4≦x≦1 which is used in pure form or in solid solution with nickel, manganese and aluminum. The main obstacles to general application of this type of electrochemistry are the scarceness of cobalt and the excessively positive potential of the transition metal oxides, with consequent safety problems for the battery.
We may also mention the compounds LixTMMXO4 in which TM represents at least one metal selected from Fe, Mn and Co, optionally replaced partially with one or more elements that have a valency between 1 and 5. These compounds only exchange lithium, and only have a very low electronic and ionic conductivity. These handicaps can be overcome by using very fine particles (such as nanoparticles) and by depositing a carbon coating by pyrolysis of organic compounds. The drawbacks associated with the use of nanoparticles are poor compactness, which is reflected in a loss of specific energy, and this problem is made worse by the deposition of carbon. Moreover, carbon deposition is carried out at high temperature, in reducing conditions. In practice, it is difficult to use elements with an oxidation state above 2, as they are reduced. This applies to FeIII, MnIII, CrIII, VuIII, VIV, which are useful dopants for increasing the ionic or electronic conductivity. Other compounds have been proposed, notably compounds corresponding to the general formula AaMb(SO4)cZd, in which A represents at least one alkali metal, Z represents at least one element selected from F and OH, and M represents at least one divalent or trivalent metal cation. These compounds comprise in particular the fluorosulfates. L. Sebastian, et al. [J. Mater. Chem. 2002, 374-377] describe the preparation of LiMgSO4F by the ceramic route. Moreover, US-2005/0163699 describes the preparation, by the ceramic route, of fluorosulfate compounds of lithium and of M in which M is Ni, Fe, Co, Mn, (MnMg), (FeZn), or (FeCo). These compounds are prepared by the ceramic route starting from the Li precursor LiF and the sulfate of the element or elements M. Among these compounds, the most interesting are those that contain Fe, because besides their relatively low cost, they are likely, on the basis of structural and chemical considerations (notably the ionocovalency of the bonds) to display interesting electrochemical properties in a desirable potential range for guaranteeing reliable use for high-volume applications. For reasons of inductive effect, the sulfates generally have higher potentials than the phosphates, whatever their structure. Examples of preparation of compounds containing various metallic elements are described in the aforementioned US-2005/0163699. Thus, example 2 describes the preparation of a compound LiFeSO4F by a ceramic method at 600° C. that gives an inhomogeneous compound, then at 500° C. with production of a reddish black compound, or at 400° C. in air with production of a red compound. This method can permit the reduction of the SO42− group by Fe2+ in the absence of oxygen according to SO42−+2Fe2+SO2+O2−+2Fe3+. The red color found in the compounds obtained at the different temperatures is due to the O2+/Fe3+ association in a crystal lattice for example in the form of the oxide Fe2O3. It is known, moreover, that the compounds of FeII oxidize in air starting from 200° C., giving FeIII, and the preparation of example 2 at 400° C. in air confirms this. The iron-containing compounds that are prepared by the ceramic route from LiF and iron sulfate according to US-2005/0163699 therefore do not consist of LiFeSO4F. Moreover, it appears that compounds in which M is Co or Ni are not stable at the temperatures employed during the recommended preparation by the ceramic route. Therefore it is not plausible that the compounds described in US-2005/0163699 had really been obtained.
The methods of preparation of alkali metal and transition metal fluorosulfate compounds by the ceramic route are generally inexpensive, but they have very slow kinetics.
FR-2 937 970 describes the preparation of lithium and transition metal sulfates, from hydrated transition metal sulfate and lithium fluoride, using a hydrophobic ionic liquid as the reaction substrate. Methods in which the precursors of the fluorosulfate are in solution or in suspension in an ionic liquid medium have faster kinetics, and they make it possible to control the crystallographic structure of the compounds obtained, as said ionic liquid has the effect of encapsulating the hydrated sulfate molecule used as precursor of the sulfate anion and of Fe. However, the use of an ionic liquid makes the method expensive because of the cost of the ionic liquids, and rather impractical because the substrate is liquid.